1. Field of the Invention
The present invention relates to a voltage current conversion device used in a light emitting device such as an exposing apparatus of an electrophotographing system and a display device, and in particular, it relates to a voltage current conversion device preferably disposed in a signal transfer channel for providing a video signal which is a voltage signal to each pixel as a current signal.
2. Related Background Art
As a light emitting device, a display device will be cited as an example. In recent years, in general, an organic electroluminescence (EL) display device, which is one of the Trendy flat display panels, includes a display device of the type in which luminous brightness of an organic EL element disposed in each pixel as a light emitting element is decided by a current value provided in a data signal source. In this type, a voltage current conversion device is disposed in the data signal source and a video signal which is a voltage signal is required to be converted into a current signal. As the voltage current conversion device, for example, its circuit constitution is illustrated in FIG. 9 of Japanese Patent Application Laid-Open No. 2002-40074.
In FIG. 3 is shown one example of the circuit constitution of the voltage current conversion device. In the drawing, reference characters T0 and T1 denote a n-type transistor, respectively, and reference character Vin denotes a voltage to be inputted, and reference character Iout denotes a current to be outputted from the circuit, and reference character V1 denotes a first potential, and reference character S1 denotes a control signal.
In the device of FIG. 3, the transistor T1 outputs a current Iout of the value corresponding to the gate voltage by a gate potential set by the voltage Vin which is inputted through the transistor T0 provided as occasion arsies.
However, in case the device of FIG. 3 is disposed in a plurality of data signal lines corresponding to a plurality of pixel columns, respectively, due to the irregularity of a threshold value Vth of the transistor T1, an irregularity occurs in the output current Iout for the input voltage Vin of the same level.
To solve such a problem, first, the present inventor invented a voltage current conversion device of the circuit constitution as shown in FIG. 4. In the drawing, reference characters T0 to T3 and T6 denote a n-type transistor, respectively, reference character T7 denotes a p-type transistor, reference characters S1 to S3 and S6 denote an independent control signal, respectively, reference characters C1 and C2 denotes capacitors, reference characters V1 and V3 denote potential sources (potential sources) providing first and third potentials, reference character Vin denotes an input terminal to be inputted with an input voltage, and reference character Iout denotes an output terminal from which an output current is outputted.
The operation of the device of FIG. 4 will be described by the timing chart of FIG. 5. In the drawing, VG(T1) shows a gate potential of the first transistor T1, and VS(T1) shows a source potential of the first transistor T1.
First, in a time t1, control signals S2 and S3 become a high level H, and the second transistor T2 and the third transistor T3 turn on. In this manner, the gate potential VG(T1) of the first transistor T1 is pre-charged by the first potential source V1.
Next, in a time t2, the control signal S3 of the gate of the third transistor T3 becomes a low level L, so that the third transistor T3 turns off and the first transistor T1 is applied with a self-bias toward a turn-on voltage, and discharges by drawing a gentle curve. In this manner, the VG(T1) discharges toward the third potential V3 until it becomes the threshold value Vth of the transistor T1 by taking a sufficient time. Here, Va′=Vth.
After the completion of the self-bias period, in a time t3, the control signal S2 becomes L, and the second transistor T2 turns off, and further, in a time t4, the control signal S1 becomes H, and the input control transistor T0 turns on. As a result, the gate of the first transistor T1 becomes a potential Vb′. Here, the Vb′ is a sum of the threshold value Vth of the first transistor T1 and the capacitor split voltages Vc of C1 and C2 of the input voltage Vin. In a time t5, the control signal S1 becomes L and the input control transistor T0 turns off.
In an appropriate time subsequent to the time t5, the control signal S6 becomes H, and the sixth transistor T6 turns on as occasion arises, and the output current Iout corresponding to the gate potential VG(T1) of the first transistor T1 which is set at the time t4 to t5 through the sixth transistor T6 and the first transistor T1 is obtained.
The voltage current conversion device of FIG. 4 has no irregularity other than the threshold value Vth of the first transistor T1, and if a sufficient self-bias is applied, the conversion device is supposed to be able to obtain a voltage current conversion characteristic which does not have an irregularity among a plurality of voltage current conversion devices.
However, in the voltage current conversion device of FIG. 4, a charge Q injected to the gate of the first transistor T1 becomes C2×(Vpre−Vth). Here, Vpre shows a voltage pre-charged to the gate of the first transistor T1 through the transistors T2 and T3 which are turned on from the first potential V1. The charge Q must be discharged during the self-bias period of the times t2 to t3. In reality, since the discharging period is finite, the gate potential VG(T1) after the self-bias becomes Va′. Here, Va is a sum of the threshold value Vth of the first transistor T1 and a self-bias residual voltage Vr. Hence, citing the application to the display device as an example, even in case a black display is made in the EL element, the current corresponding to the self-bias residual voltage Vr ends up flowing into the pixel having the EL element. That is, the current supply to the pixel is completely shut off, and the black display is unable to be performed, thereby lowering a display contrast.